VANCOUVER/TORONTO, Jan 30 (Reuters) - A massive spill from a dam containing mine waste in British Columbia last year was caused by a flawed design for the embankment, which did not account for the presence of a glacial lake deposit at the foundation, an independent panel said on Friday.

The probe found that the breach at Imperial Metal's Mount Polley mine, which sent billions of gallons of wastewater and sludge into waterways, happened because the dam's weight was too much for the foundation to bear.

"We concluded the dominant contribution to the failure resides in the design," the panel's chair, Norbert Morgenstern, said after the 5-1/2-month investigation.

"The design did not take into account the complexity of the sub-glacial and pre-glacial geological environment associated with the perimeter embankment foundation."

The panel, appointed by the provincial government with the backing of two Aboriginal bands from the Mount Polley area, also found the collapse was triggered by the construction of a downstream rockfill zone at an overly steep slope.

They concluded that had the slope been flattened, work that was under way when the accident happened, the failure could have been averted.

Morgenstern said design flaws created a "loaded gun" and the construction of the steep slope "pulled the trigger."

Knight Piesold Consulting designed the tailings pond, but ended its service as "engineer of record" for the facility in early 2011. The firm was not immediately available for comment on Friday.

Vancouver-based Imperial Metals' shares closed up 6 percent at C$8.95 on the Toronto Stock Exchange. The stock, which plunged more than 46 percent the day after the Aug. 4, spill, hit a near five-year low of C$7.30 in December.

The 147-page report had seven recommendations for industry and government, including stronger regulatory overview and emphasis on safety over economics in the feasibility stage of mining projects.

British Columbia's energy and mines minister said all mines operating in the province will be immediately required to report on whether their tailings dams are built on glacial material.

The massive spill had called into question provincial government inspections of tailings dams and could delay or even derail other energy and mining projects planned in the famously "green" province.

However, the panel found that additional inspections of the tailings facility would not have prevented the failure.

The Mount Polley copper and gold mine is near the city of Williams Lake, some 550 km (340 miles) northeast of Vancouver. (Additional reporting by Nicole Mordant in Vancouver; Editing by G Crosse and Jonathan Oatis)